17-5 Nuclear Energy

How Does a Nuclear Fission Reactor Work? Splitting Nuclei to Produce Electricity

In a nuclear reactor, a material known as a moderator slows down enough of the fast-moving fission neutrons so that the neutrons from one fission reaction subsequently cause additional fission reactions—a process called a chain reaction. The high-temperature heat resulting from the chain reaction is carried away from the reactor by a coolant and is used to produce high-pressure steam, which spins turbines that generate electricity.

The Canadian design is called a CANDU reactor, an acronym derived from the words “Canada,” “deuterium,” and “uranium”.

Unlike other water-cooled reactors, the CANDU design uses heavy water as coolant and moderator. In heavy water, the hydrogen atoms are replaced with atoms of deuterium, which is an isotope of hydrogen having one extra neutron. The result is water that is about 11% heavier that normal and has a greater ability to slow down fast-moving fission neutrons.

There are 22 large reactors in Canada—all of CANDU design. Twenty are in Ontario; Quebec and New Brunswick have one each. Together, the reactors produce 12% of Canada’s electricity. Ontario is Canada’s “nuclear province” and produces 30–50% of its electricity this way.

What Went Wrong at Chernobyl? Human Error Was at the Root of the Problem

There are several versions of the Chernobyl story that differ in details; however, all versions agree that on April 25, 1986, the reactor crew at Chernobyl Unit-4 was scheduled to shut down the reactor. Instead of simply shutting it down, they performed an experiment to determine how long the turbines would continue to spin and generate electricity following the loss of the main power supply. This experiment had already been performed successfully at Chernobyl and other power plants.

What Is the Nuclear Fuel Cycle? Looking at the Whole Picture

Nuclear power plants, each with one or more reactors, are only one part of the nuclear fuel cycle. Unlike other energy resources, nuclear energy produces high-level radioactive wastes that give off large amounts of harmful ionizing radiation for a short time and small amounts for a long time.

After approximately 15–60 years of operation, a nuclear reactor becomes dangerously contaminated with radioactive materials, and many of its parts become brittle or corroded and worn out. Unless the plant’s life can be extended by expensive renovation, it must be decommissioned or retired.

In the closed nuclear fuel cycle, the fissionable isotopes uranium-235 and plutonium-239 are removed from spent fuel assemblies for reuse as nuclear fuel.
In the open nuclear fuel cycle the isotopes are not removed by reprocessing the nuclear wastes and are eventually buried in an underground disposal facility.

How Did We Get into Nuclear Power, and How Successful Has It Been? A Technology in Question

In the 1950s, researchers projected that by the year 2000 at least 1 800 nuclear power plants would supply 21% of the world’s commercial energy and most of the world’s electricity.

After almost 60 years of development, enormous government subsidies, and an investment of more than $2 trillion (U.S.) worldwide, these goals have not been met. Instead, by 2010, 436 commercial nuclear reactors in 31 countries were producing only 6% of the world’s commercial energy and 14% of its electricity.

According to energy analysts and economists, there are several major reasons for the failure of nuclear power to grow as projected. They include multibillion-dollar construction cost overruns, higher operating costs and more malfunctions than expected, and poor management. Two other major setbacks have been public concerns about safety and stricter government safety regulations.

What Are the Advantages and Disadvantages of the Conventional Nuclear Fuel Cycle? Better Than Coal, but There Are Other Problems

Some proponents of nuclear power claim it will help reduce dependence on imported oil. But other analysts point out that nuclear power has little effect on oil use because burning oil typically produces only a small amount of electricity. The major use for oil is to produce gasoline and diesel fuel for transportation, which would not be affected by nuclear power unless we switch to electric cars in the future.

Proponents say we should increase the use of nuclear power because its use does not release the greenhouse gas carbon dioxide into the atmosphere. It is true that nuclear power plants do not release carbon dioxide. However, the nuclear fuel cycle does release this gas into the atmosphere, although emissions are about one-sixth per unit of energy compared to burning fossil fuels.

How Vulnerable Are Nuclear Power Plants to Disasters of Various Kinds? A Serious Concern

The 2001 destruction of New York City’s World Trade Center towers has raised fears that a similar attack could break open a reactor’s containment shell and set off a reactor meltdown that could create a major radio- active disaster.

Nuclear officials say such concerns are overblown and that nuclear plants could survive such an attack because of the thickness and strength of the containment walls. But a 2002 study by the Nuclear Control Institute found that the plants were not designed to withstand the crash of a large jet travelling at the impact speed of the two hijacked airliners that hit the World Trade Center. Further tests revealed that there is insufficient security at nuclear plants to withstand ground-level attacks by terrorists.

How Do We Dispose of Low-Level Radioactive Waste? Manage It on Site or Store It Safely

Each part of the nuclear fuel cycle produces low-level and high-level solid, liquid, and gaseous radioactive wastes with various half-lives.

Wastes classified as low-level radioactive wastes give off small amounts of ionizing radiation and must be stored safely for 100–500 years before decaying to safe levels. Such wastes include tools, building materials, clothing, glassware, and other items that have been contaminated by radioactivity.

According to Natural Resources Canada, the nuclear and other industries have produced 2.3 mil- lion m3 of low-level wastes in Canada (as of 2004). These waste materials are stored in specially designed aboveground buildings while awaiting a more permanent disposal option. There are plans to build a deep-disposal site at the Bruce Nuclear Plant for low-level and medium-level wastes.

How Do We Dispose of High-Level Radioactive Waste? An Unresolved Issue

After more than 50 years of research, scientists still do not agree on whether there is a safe, permanent solution for dealing with high-level radioactive waste. Here are some of the proposed methods and their possible drawbacks:

Shoot it into space or into the sun: Costs would be very high, and a launch accident could disperse high-level radioactive wastes over large areas of the Earth’s surface. This strategy has been abandoned for now
Bury it under the Antarctic ice sheet or the Greenland ice cap. The long-term stability of the ice sheets is not known. They could be destabilized by heat from the wastes, and retrieving the wastes would be difficult or impossible if the method failed.
Dump it into descending subduction zones in the deep ocean. But wastes eventually might be spewed out somewhere else by volcanic activity, and containers might leak and contaminate the ocean before being carried downward. Also, retrieval would be impossible if the method did not work.
Bury it in thick deposits of mud on the deep-ocean floor in areas that tests show have been geologically stable for 65 million years. The waste containers eventually would corrode and release their radioactive contents.
Change it into harmless, or less harmful, isotopes. Scientists are investigating the use of a particle accelerator to transform long-lived fission wastes into other shorter-lived radioisotopes (by proton absorption, for example), thereby reducing the amount of time for which wastes must be safely stored. However, there is currently no efficient way to do this.
Bury it deep underground. This favoured strategy is under study by all countries producing nuclear waste. Many experts feel that the geologic repository option is the only scientifically credible long-term solution for safely isolating such wastes. This option raises a number of issues: where to locate such a site, how to safely transport the radioactive waste, how to avoid unforeseen long-term problems and how to gain public acceptance.

What Can We Do with Worn-Out Nuclear Plants? A Hidden Cost

When a nuclear plant comes to the end of its useful life, it must be decommissioned. Scientists have proposed three ways to do this:

One is to dismantle the plant and store its large volume of highly radioactive materials in a high-level nuclear waste storage facility. The safety of this option is questioned by a number of scientists.
A second approach is to mothball the plant by erecting a physical barrier and setting up full-time security for 30–100 years before the plant is dismantled. This allows time for some of the radioactive mate- rial to decay to levels that make dismantlement safer.
A third option is entombment, which involves enclosing the entire plant in a concrete tomb that must last and be monitored for several thousand years. A drawback of this option is that concrete breaks down after a comparatively short period.

Can We Afford Nuclear Power? Financial Considerations

Experience has shown that the nuclear power fuel cycle is an expensive way to produce electricity, even when huge government subsidies partially shield it from free-market competition with other energy sources.

Partly to address cost concerns, the nuclear industry plans to build smaller second-generation plants using standardized designs, which it claims are safer and can be built more quickly (in 3–6 years). These new designs would have built-in passive safety features designed to make explosions or the release of radioactive emissions highly unlikely.

Is Breeder Nuclear Fission a Feasible Alternative? An Uncertain Technology

Some nuclear power proponents urge the development and widespread use of breeder nuclear fission reactors, which generate more nuclear fuel than they consume by converting nonfissionable uranium-238 into fissionable plutonium-239. Because breeders would use more than 99% of the uranium in ore deposits, the world’s known uranium reserves would last at least 1 000 years, and perhaps several thousand years.

However, if the safety system of a breeder reactor fails, the reactor could lose some of its liquid sodium coolant, which ignites when exposed to air and reacts explosively if it comes into contact with water. This could cause a runaway fission chain reaction and perhaps a nuclear explosion powerful enough to blast open the containment building and release a cloud of highly radioactive gases and particulate matter.